Accurate colour editing and reproduction is a mature technology in the field of two dimensional (2D) printing, especially for diffuse colour reproduction and printing. However, reproduction of only colour does not allow representing a very wide range of material appearances, including reflective properties such as for shiny metallic surfaces. The sole use of colour to represent a reflective characteristic such as a shiny metallic surface leads to a dull plastic appearance when under varying illumination and viewing conditions. Recent 2D printing systems have added further capabilities to control optical properties of the printed surface, including but not limited to angular-dependent reflection properties of the print and translucency.
Furthermore, in recent years, 2.5 dimensional (2.5D) and three dimensional (3D) printing technologies have emerged. 2.5D printers allow printing a limited height relief, similar to the height relief of oil paintings, and 3D printers allow printing objects with arbitrary shapes. 3D printing has been mainly used as a fast prototyping tool for functional purposes where accuracy of the shape is most important but colour or texture bears minor to no importance. In contrast, 2.5D printing focuses on printing applications where the aesthetic aspect of the surface is of high importance. In 2.5D printing, the appearance of the surface of the object plays a crucial role in the perception and value of the object. Characteristics affecting surface appearance, such as diffuse colour, highlight/reflection colour, glossiness, roughness and colour travel, impact the user's perception of the appearance of that object or surface. Current applications of such technology include artwork reproduction, design, and high-quality packaging, where appearance can vary locally, e.g. from matte/dull to shiny/glossy.
For example, artwork reproduction of oil paintings is used for educational purposes and requires a precise replication of surface texture and gloss to recreate the artist's original painting. Cultural heritage can be digitally preserved by 3D scanning the art object and requires the scanning of not only the colour information of the surface of the object but also of the precise relief of the surface and light reflectance characteristics of the surface. An oil painting typically exhibits a certain gloss that contributes to the artistic intent and therefore needs to be captured in the digital scan, and reproduced physically if the scanned object is printed. The scanning of the object is not the field of this invention. Once the object is digitally imported into a computer, it then needs to be digitally processed before printing. Colours and other appearance aspects of the surface may need adjustment. In another example of object design, the user designs an object and its appearance using a computer-aided design (CAD) software tool and wants to manipulate the surface appearance of the object to a desired effect, such as giving a glossy metallic effect to the surface.
Colour editing and management is a known practice in the printing industry workflow. However, controlling additional aspects related to the optical characteristics of the surface is still a technical challenge. In general, designers rely on CAD software tools to produce or reproduce a desired surface appearance, sometimes termed ‘look and feel’.
In a typical scenario, a user wants to design an object and the object's surface appearance, for example a piece of jewellery with a metallic golden reflection aspect. A computer-aided design software tool is often used to design the shape of the object in the form of a 3D mesh of polygons. The same software or different software is used to apply a texture on the 3D mesh and to manipulate the surface appearance. The texture, with specific geometric variations, can be chosen from a library of examples to be applied on the surface of the object. Parameters related to geometry of the surface, such as bumpiness, randomness, and frequency, are set by the user. Additionally, parameters related to the behaviour of the surface in relation to light reflections can be set by the user. Parameters related to perceived surface appearance, such as diffuse colour, reflectivity, roughness, gloss are manually set by the user until satisfied by the appearance as simulated on the computer monitor. Each parameter is controlled independently from all other parameters. In particular, surface geometry is controlled independently to the reflectance characteristics of the surface. In conventional tools, knowing which parameter(s) to modify and how to modify the parameter(s) requires a high level of expertise and experience with such tools. The parameters are either directly mapped to mathematical parameters in the rendering model and are therefore not intuitive to understand in terms of their effect on surface appearance, or are high-level concepts that require the user's understanding and often do not map directly to the rendering model that drives the modification of the reflectance properties of the material. Furthermore, the parameters do not relate to the mesoscale geometry of the surface, which can affect the perceived appearance, for example perceived gloss. Perceived gloss or glossiness is the perceptual response of the human visual system processing the light information related to physical gloss coming from the object's surface. Perceived gloss is related to the capability of the human visual system to distinguish between diffuse and specular light information coming from the surface, and the interpretation by the human visual system of the information related to these two components. This interpretation depends for example on the sharpness of specular reflections.
In a scenario where the object is intended to be printed, it is desirable for the rendering system to provide a preview of the result of the print. In such case, the simulated material is digitally displayed so that the user can have a precise idea of the finished state of the edited material. The user previews the edited material in order to judge and confirm the target appearance, including perceived colour and reflectance properties of the surface. The function is important to reduce the number of printing trials and errors otherwise needed by the user to obtain the desired printed appearance. In the absence of a preview function, the user needs to print the current edited material, and confirm if the result is as the user desired. If this is not the case, the user needs to modify some material appearance settings, print again and visually confirm again if the printing result matches the user's expectation. The development process can therefore be time-consuming and expensive to achieve a desired material appearance. A preview function can reduce substantially the time and cost of achieving a desired printed material appearance.
As 3D and 2.5D printing become more widely available, the need to create and modify the appearance of materials is spreading to a wider range of users, often not specialised or familiar with graphics parameters.